Pure rotational spectra of the ClOO radical for the Cl-35 and Cl-37 isotopomers have been observed using Fourier transform microwave and Fourier transform microwave-millimeter wave double resonance spectroscopy. The rotational, centrifugal, spin-rotation coupling, and hyperfine coupling constants have been determined by least-squares fits of the observed transition frequencies. The molecular constants indicate that the electronic ground state is (2)A". The r(0) structure is determined to be r(0)(ClO)=2.075 Angstrom, r(0)(OO)=1.227 Angstrom, and theta(0)(ClOO)=116.4degrees. Several highly accurate ab initio calculations have also been performed. Some of them turned out to be inaccurate because it is necessary to take into account both static and dynamic electronic correlations. Only multireference (single and double) configuration interaction calculations with large basis sets reproduce the present experimental results. The anharmonic force constants obtained by the ab initio calculations are used to determine the r(e) structure, r(e)(ClO)=2.084(1) Angstrom, r(e)(OO)=1.206(2) Angstrom, and theta(e)(ClOO)=115.4(1)degrees. Unique features of the ClOO radical have become clear by the present experiment and the ab initio calculations. (C) 2004 American Institute of Physics.